Nowadays, commercialization of driving safety support systems utilizing an on-board communication device that performs car-to-car communication is being considered. In this case, an information exchange application that transmits/receives information of own car between cars per certain cycle is typically used in the on-board communication device. Moreover, in the car-to-car communication system, it is known that a carrier sense multiple access (CSMA) system has been conventionally used as an access system such that each car transmits information by itself.
When an information exchange application is used in the CSMA system, in a case where the number of cars positioned in a communication area increases, communication traffic increases to exceed a communication capacity. Therefore, it is conceivable that congestion in which the reliability of communication deteriorates may occur, information through car-to-car communication may not be transmitted reliably, and accordingly safety support services cannot be provided.
Patent Document 1 discloses, in order to prevent congestion from occurring in a car-to-car communication system, the method of avoiding congestion by controlling a cyclic interval (controlling an information amount) of own car based on a dangerous situation of a car and a traffic amount of a communication path.
Further, it is assumed that in addition to an information exchange application, multiple applications such as an application that transmits emergency information will be used in a car-to-car communication system. Moreover, in order to provide those applications, messages need to be retransmitted or divided and assembled.
Therefore, the method of using a local port control protocol (LPCP) for correspondence with a multi-application and using a local port protocol (LPP) for correspondence with retransmission or division and assembly of a message in a road-to-vehicle communication system is disclosed in Patent Document 2 and the standard “dedicated short-range communication (DSRC) application sub layer ARIB STD-T88” defined by the association of radio industries and businesses (laid down May 25, 2004).    Patent Document 1: Japanese Patent Application Laid-Open No. 2006-209333    Patent Document 2: WO 2005-039075